Critical dimension variation correction in extreme ultraviolet lithography

ABSTRACT

A method of correcting a critical dimension (CD) variation in extreme ultraviolet (EUV) photolithography includes mapping the CD variation of a wafer exposure field formed by a photolithography system that includes an EUV photolithography photomask. Parameters of a treatment to produce a change in reflectance at a working wavelength of EUV radiation in a region of a reflective multilayer of the photomask are determined, the change in reflectance being calculated to correct the mapped CD variation. A treatment beam is directed to the region. The region is treated with the beam in accordance with the determined parameters.

CROSS REFERENCE TO RELATED APPLICATIONS

The present invention is a continuation application of U.S. patentapplication Ser. No. 15/189,058, filed on Jun. 22, 2016 and published asUS Patent Application Publication No. 2016/0370697 on Dec. 22, 2016, andwhich claims the priority benefit of Israeli Patent Application No.239577, filed on Jun. 22, 2015, all of which are incorporated in theirentirety herein by reference.

FIELD OF THE INVENTION

The present invention relates to a method and system for correctingcritical dimension variations in extreme ultraviolet lithography.

BACKGROUND OF THE INVENTION

As miniaturization of integrated circuits continues, photolithographyprocesses that use deep ultraviolet (DUV) radiation are ofteninadequate, typically at the wavelength of 193 nm. In order to supportthe further miniaturization, radiation with a wavelength that is anorder of magnitude shorter than that of DUV radiation is required.

Developers of photolithography processes have turned to extremeultraviolet (EUV) radiation. For example, EUV photolithography mayutilize radiation at the wavelength 13.5 nm. A typical 13.5 nm radiationsource includes a plasma (e.g., a laser-produced plasma) in which theradiation is produced.

A photolithography system based on EUV radiation differs in severalaspects from a system based on DUV radiation. A DUV system typicallyinvolves transmitting optics (e.g., lenses). A DUV photomask typicallyinvolves a transparent substrate upon which a pattern of reflecting orabsorbing material is deposited. Since EUV radiation is absorbed bymaterials, all EUV lithography optics are based on reflection, ratherthan transmission and refraction. Typical EUV optics is based onmultilayer reflectors that reflect radiation as determined by Bragg'sLaw. The multilayer reflectors typically include a large number (e.g.,40 to 50, or more or fewer) of alternating layers of molybdenum (Mo) andsilicon (Si). The reflectors are used in focusing and beam-directingoptics, as well as in the photomask itself. A typical EUV photomaskincludes reflector upon which an absorbing material is deposited.

Process variations in lithographic printing of integrated circuits arecharacterized by variations in the sizes of structures (e.g., lines andspaces) printed on a substrate such as a silicon wafer. Variations ofprinted lines and spaces are measured as an average critical dimension(CD) size at different areas of the wafers. Variations in the width oflines and spaces imprinted on a wafer may occur, for example, due tophotomask imperfections. Other sources of CD variations on the wafer mayinclude, for example, unevenness on a chuck that holds a wafer orvariations in the photolithography process.

SUMMARY OF THE INVENTION

There is thus provided, in accordance with an embodiment of the presentinvention, a method of correcting a critical dimension (CD) variation inextreme ultraviolet (EUV) photolithography, the method including:mapping the CD variation of a wafer exposure field formed by aphotolithography system that includes an EUV photolithography photomask;determining one or a plurality of parameters of a treatment to produce achange in reflectance at a working wavelength of EUV radiation in aregion of a reflective multilayer of the photomask, the change inreflectance being calculated to correct the mapped CD variation;directing a treatment beam to the region; and treating the region withthe beam in accordance with the determined one or a plurality ofparameters.

Furthermore, in accordance with an embodiment of the present invention,the treatment beam includes a beam of a continuous wave or pulsed laser.

Furthermore, in accordance with an embodiment of the present invention,the pulsed laser includes a femtosecond pulsed laser.

Furthermore, in accordance with an embodiment of the present invention,a parameter of the one or a plurality of parameters is selected from agroup of parameters consisting of pulse energy, pulse duration, pulserate, exposure time, scan rate, focal length, distance, and pitch of ascanning pattern.

Furthermore, in accordance with an embodiment of the present invention,determining the one or a plurality of parameters includes utilizing aresult of a calibration that includes treating a plurality of regions ofa reflective multilayer surface in accordance with different values ofthe one or a plurality of parameters.

Furthermore, in accordance with an embodiment of the present invention,the working wavelength is substantially equal to 13.5 nm.

Furthermore, in accordance with an embodiment of the present invention,the produced change in reflectance of the working wavelength includeschanging a peak value of a reflectance spectrum of the region orwavelength shifting of a peak of a reflectance spectrum of the region.

Furthermore, in accordance with an embodiment of the present invention,directing the beam to the region includes translating a source of thebeam or the photomask, or operating beam optics.

Furthermore, in accordance with an embodiment of the present invention,treating the region with the beam includes scanning the beam across theregion.

Furthermore, in accordance with an embodiment of the present invention,scanning the beam includes scanning the beam with a raster pattern.

Furthermore, in accordance with an embodiment of the present invention,the raster pattern includes a sequence of overlapping laser spots.

Furthermore, in accordance with an embodiment of the present invention,a ratio of a diameter of a laser spot of the sequence of overlappinglaser spots to a pitch of the raster pattern is in the range of 1 to100.

Furthermore, in accordance with an embodiment of the present invention,an energy density of the treatment ranges from 10 mJ/cm² to 10 J/cm².

Furthermore, in accordance with an embodiment of the present invention,the energy density is less than 200 mJ/cm².

Furthermore, in accordance with an embodiment of the present invention,mapping the CD variation includes examination of a wafer that is printedusing the photomask, examination of the photomask, or sensing a patternof radiation in the photolithography system.

Furthermore, in accordance with an embodiment of the present invention,the treatment beam heats the region.

There is further provided, in accordance with an embodiment of thepresent invention, a photomask including: a multilayer on a substrate,the multilayer being reflective at a working wavelength of EUVradiation, one or a plurality of regions of the multilayer being locallytreated to modify a reflectance at the working wavelength of a region ofthe one or a plurality of regions so as to compensate for a mappedcritical dimension (CD) variation; and a pattern of a material that isabsorbing at the working wavelength, the pattern being on themultilayer.

Furthermore, in accordance with an embodiment of the present invention,the multilayer includes a plurality of alternating layers silicon andmolybdenum.

Furthermore, in accordance with an embodiment of the present invention,the one or a plurality of regions are locally treated by heating.

There is further provided, in accordance with an embodiment of thepresent invention, a system for correcting a CD variation in EUVphotolithography, the system including: a radiation source to produce atreatment beam; and a controller configured to direct the treatment beamat a region of a reflective multilayer of an EUV photolithographyphotomask and to operate the source to treat the region with the beam inaccordance with one or a plurality of parameters to produce a change inreflectance at a working wavelength of EUV radiation in the region, thechange in reflectance being calculated to correct a mapped CD variation.

Furthermore, in accordance with an embodiment of the present invention,the treatment beam is configured to irradiate the region via a substrateof the photomask.

Furthermore, in accordance with an embodiment of the present invention,the system includes optics to direct, focus, filter or collimate thebeam.

Furthermore, in accordance with an embodiment of the present invention,the optics are configured to scan the beam over the region.

Furthermore, in accordance with an embodiment of the present invention,the radiation source includes a continuous wave or pulsed laser.

Furthermore, in accordance with an embodiment of the present invention,the system is configured to produce relative translation between theradiation source and the photomask.

Furthermore, in accordance with an embodiment of the present invention,the system is configured to heat the region with the treatment beam.

There is further provided, in accordance with an embodiment of thepresent invention, a method for reducing reflection by a black border ofa photomask for EUV photolithography, the method including: directing atreatment beam to a reflective multilayer of the photomask at the blackborder; and treating the region with the beam so as to reduce reflectionof the multilayer to at least a predetermined reflectance value.

Furthermore, in accordance with an embodiment of the present invention,the treatment beam is directed to the reflective multilayer through abackside coating and a photomask substrate.

Furthermore, in accordance with an embodiment of the present invention,the predetermined reflectance value is 5%.

Furthermore, in accordance with an embodiment of the present invention,the treatment beam heats the reflective multilayer.

BRIEF DESCRIPTION OF THE DRAWINGS

In order for the present invention, to be better understood and for itspractical applications to be appreciated, the following Figures areprovided and referenced hereafter. It should be noted that the Figuresare given as examples only and in no way limit the scope of theinvention. Like components are denoted by like reference numerals.

FIG. 1 schematically illustrates an extreme ultraviolet (EUV)photolithography system that utilizes critical dimension (CD) variationcorrection, in accordance with an embodiment of the present invention.

FIG. 2A schematically illustrates a system for treating a reflectivephotomask for EUV photolithography to correct for critical dimensionvariation, in accordance with an embodiment of the present invention.

FIG. 2B schematically illustrates components of a variation of thesystem of FIG. 2A.

FIG. 3 schematically illustrates modification of reflectance of a regionof a reflective multilayer that is treated to correct for CD variation,in accordance with an embodiment of the present invention.

FIG. 4 schematically illustrates a reflective multilayer surface forcalibration of treatment to correct for CD variation, in accordance withan embodiment of the present invention.

FIG. 5 is a flowchart illustrating a method for correcting CD variationsin EUV lithography, in accordance with an embodiment of the presentinvention.

FIG. 6 schematically illustrates a layout of a photomask with a blackborder to which a reflection reducing treatment is applicable, inaccordance with an embodiment of the present invention.

FIG. 7 schematically illustrates application of a treatment for reducingreflectance of a black border, in accordance with an embodiment of thepresent invention.

FIG. 8 is a flowchart illustrating a method for reducing reflectance ofa black border, in accordance with an embodiment of the presentinvention.

DETAILED DESCRIPTION OF THE INVENTION

In the following detailed description, numerous specific details are setforth in order to provide a thorough understanding of the invention.However, it will be understood by those of ordinary skill in the artthat the invention may be practiced without these specific details. Inother instances, well-known methods, procedures, components, modules,units and/or circuits have not been described in detail so as not toobscure the invention.

Although embodiments of the invention are not limited in this regard,discussions utilizing terms such as, for example, “processing,”“computing,” “calculating,” “determining,” “establishing”, “analyzing”,“checking”, or the like, may refer to operation(s) and/or process(es) ofa computer, a computing platform, a computing system, or otherelectronic computing device, that manipulates and/or transforms datarepresented as physical (e.g., electronic) quantities within thecomputer's registers and/or memories into other data similarlyrepresented as physical quantities within the computer's registersand/or memories or other information non-transitory storage medium(e.g., a memory) that may store instructions to perform operationsand/or processes. Although embodiments of the invention are not limitedin this regard, the terms “plurality” and “a plurality” as used hereinmay include, for example, “multiple” or “two or more”. The terms“plurality” or “a plurality” may be used throughout the specification todescribe two or more components, devices, elements, units, parameters,or the like. Unless explicitly stated, the method embodiments describedherein are not constrained to a particular order or sequence.Additionally, some of the described method embodiments or elementsthereof can occur or be performed simultaneously, at the same point intime, or concurrently. Unless otherwise indicated, us of the conjunction“or” as used herein is to be understood as inclusive (any or all of thestated options).

In accordance with an embodiment of the present invention, a reflectivemultilayer of an extreme ultraviolet (EUV) photomask is treated with acontinuous wave (CW) laser, a pulsed laser (e.g., an ultrafast pulsedlaser) or otherwise heated. The heating may modify the reflectiveproperties of the reflector for EUV radiation having a particularworking photolithography wavelength (e.g., 13.5 nm or anotherwavelength). For example, the selected working photolithographywavelength of EUV radiation may be a wavelength that is useful inapplication of EUV photolithography for printing on a wafer to producean integrated circuit or other electronic component. Modification of themultilayer reflector may be utilized to compensate for criticaldimension (CD) variation. The heat treatment may reduce the reflectanceof the multilayer reflector at the working photolithography wavelength(e.g., as measured for a particular incidence or reflectance angle).Thus, reduced reflectance may compensate for CD variations that weredetected in the reflective photomask prior to the treatment. Therequired reduction in reflection may be determined in accordance with ameasured degree of CD variation. Reducing the intensity of the reflectedEUV radiation in a region of the photomask may (e.g., where the CD isabove a target value) reduce the CD in that region.

CD variations in a photolithography system that includes the reflectivephotomask may be mapped, detected, or measured. The mapping may bedirect, e.g., by examining results of wafer that was produced by aphotolithography process using the photo system with the photomask.Scanning electron micrography (SEM) or other examination of the printedwafer of a treated substrate may reveal one or more CD variations. Thepattern of radiation to which the wafer is exposed may be referred to asa wafer exposure field of, the photolithography system, or a waferexposure field that is formed by the photolithography system.Alternatively, or in addition, the mapping may be indirect. For example,the photomask itself may be examined, or a pattern of radiation alongthe optical path of the photolithography system that incorporates theEUV photomask may be sensed. The results of such indirect mappingmeasurements may be incorporated into a model (e.g., ray trace, fieldcalculation, exposure simulation, or other model) to predict the effecton a that is produced by the photolithography system with the photomask

CD variations may arise from non-uniformity of the photomask itself,from non-uniform illumination of the photomask, or from other factorsthat affect a pattern of EUV radiation in an EUV photolithography systemthat incorporates the photomask. Non-uniform illumination may resultfrom non-uniformity of the illumination source (e.g., laser-producedplasma that emits radiation at the working photolithography wavelength)or from other components of the system optics (e.g., the reflectivesurfaces used to direct and focus the illumination duringphotolithography). In some cases, the combination of variouscontributions to the CD variations may reinforce one another, or maypartially or completely cancel one another out. Thus, it may benecessary to take all contributions into consideration when mapping theCD variations.

Parameters of a treatment process in which the multilayer reflector istreated to attain a compensating reduction reflectance may be determinedby a calibration process. For example, different areas of a multilayerreflector may be treated differently and the results may be evaluated.For example, different areas of the multilayer reflector may be exposedto laser radiation when parameters of operation of the laser are varied.For example, the variations may result in irradiation of differentregions of the multilayer reflector while operational parameters of thelaser differ in one or more ways. Such irradiation differences mayinclude differing rates of irradiation (e.g., different pulse energydensity of a pulsed laser, scan rate, pitch of a scan pattern or rasterpattern during irradiation), or different quantities of totalirradiation (e.g., exposure time, exposure dose).

Results of exposure during the calibration process may be examined indifferent ways. Reflectance measurements, or spectral reflectancemeasurements, may reveal changes in reflectance at the workingphotolithography wavelength, or in a spectral region that includes theworking photolithography wavelength. A spectral reflective measurementmay reveal wavelength shifts in a reflectance spectrum that includes theworking photolithography wavelength.

FIG. 1 schematically illustrates an extreme ultraviolet (EUV)photolithography system that utilizes critical dimension variationcorrection, in accordance with an embodiment of the present invention.

EUV photolithography system 10 is configured to expose photoresist 16 onsubstrate 14 with EUV radiation that is patterned by absorber pattern 22of reflective photomask 19. Absorber pattern 22 includes lines or otherfeatures that are fashioned of a material that is absorbing to EUVradiation at least at a working photolithography wavelength. Features ofabsorber pattern 22 are configured to shade and prevent exposure of acorresponding pattern on photoresist 16 to the EUV radiation. Exposureor lack of exposure to the EUV radiation may affect the susceptibilityto removal of regions of photoresist 16 during later exposure to adeveloper solution or material.

EUV radiation beam 18 is produced by EUV radiation source 12. Forexample, EUV radiation source 12 may include a laser-produced plasmasource, discharge-produced plasma source, free-electron laser, or otherdevice that is capable of generating EUV radiation. EUV radiation source12 may include collimating or filtering optics to produce EUV radiationbeam 18 of narrowband radiation at (e.g., centered about) the workingphotolithography wavelength. The working photolithography wavelength maybe 13.5 nm, or another suitable EUV wavelength. For example, optics ofEUV radiation beam 18 may include one or more selective mirrors (e.g.,multilayer mirrors) that selectively reflect radiation of a particularwavelength into a particular direction (e.g., due to the Bragg effect).

The optical path of EUV radiation beam 18 may include EUV optics 28(collectively and schematically represented by a concave multilayeredreflecting surface). EUV optics 28 may include one or more grouped orphysically separated components that are configured to direct, focus,filter, collimate, absorb, or otherwise optically modify EUV radiationbeam 18. Although EUV optics 28 are schematically represented in theoptical path of EUV radiation beam 18 between reflective photomask 19and photoresist 16, some components of EUV optics 28 may be placedelsewhere, e.g., between EUV radiation source 12 and reflectivephotomask 19, or elsewhere in the optical path of EUV radiation beam 18.

Reflective photomask 19 includes reflective multilayer 20. Reflectivemultilayer may include a plurality (e.g., 40 to 50, or another number)of bi-layers. For example, each bi-layer may include a layer ofmolybdenum and a layer of silicon. A typical bi-layer for selectivereflection of 13.5 nm radiation may have a thickness of about 6.9 nm. Insome cases, a topmost (e.g., exposed) layer may include anothermaterial, e.g., ruthenium.

Reflective multilayer 20 may include one or more treated multilayerregions 24. (It should be noted that an actual treated multilayer region24 typically is much larger than an individual feature of absorberpattern 22, and typically many such features are included within thearea of a single treated multilayer region 24.) A treated multilayerregion 24 may have been locally treated (e.g., by laser radiation orotherwise) so as to modify the reflection of EUV radiation at theworking photolithography wavelength. For example, the treatment mayaffect a local structure of reflective multilayer 20. The localstructure may be affected by one or more mechanical or structuraleffects (e.g., expansion, contraction, warping, or other mechanical orstructural effects), by chemical effects (e.g., heat induced chemicalinteractions among components of reflective multilayer 20, or otherchemical effects), or other effects. The modification of local structuredue to the treatment may modify the optical properties of treatedmultilayer region 24. The modification of the optical properties mayinclude reducing the reflectance at the working photolithographywavelength of treated multilayer region 24 relative to the reflectanceat the working photolithography wavelength of untreated regions ofreflective multilayer 20. (When the reflectance of reflective multilayer20 is not optimized for the working photolithography wavelength, thetreatment may result in an increase in reflectance at the workingphotolithography wavelength.)

A treated multilayer region 24 may not have well-defined boundaries.Thus, the reflectance at the working photolithography wavelength mayvary gradually across a transition from treated multilayer region 24 toa neighboring untreated region of reflective multilayer 20. Furthermore,adjacent treated multilayer regions 24 may partially overlap oneanother, with the reflectance at the working photolithography wavelengthvarying from one of the adjacent treated multilayer regions 24 to theother.

Photomask substrate 26 may be formed of a material with a lowcoefficient of thermal expansion (e.g., a material based on fusedsilica, or another material with a low coefficient of thermalexpansion). The low coefficient of thermal expansion may ensure thatabsorber pattern 22 does not appreciably expand or contract (e.g., theexpansion or contraction is too small to effect operation of a componentthat is produced as a result of the photolithography process) as aresult of temperature changes that may be expected during EUVphotolithography.

When EUV radiation beam 18 is reflected from treated multilayer region24, the reduced reflection may compensate for one or more previouslydetected CD variations in irradiating photoresist 16. For example, suchCD variation may result from inaccuracies caused by EUV radiation source12, absorber pattern 22, EUV optics 28, or another component ofreflective photomask 19 or EUV photolithography system 10.

For example, EUV photolithography system 10 may include or communicatewith an inspection unit 29. Inspection unit 29 may include one or moredevices that are configured to inspect or examine a wafer print that isproduced using reflective photomask 19.

One or more regions of reflective multilayer 20 may be heated, orotherwise treated, to form one or more treated multilayer regions 24.

FIG. 2A schematically illustrates a system for treating a reflectivephotomask for EUV photolithography to correct for critical dimensionvariation, in accordance with an embodiment of the present invention.

Photomask treatment system 30 is configured to treat a region ofreflective multilayer 20 of reflective photomask 19 to form a treatedmultilayer region 24. Photomask treatment system 30 is configured totreat one or more regions of reflective multilayer 20 to form treatedmultilayer regions 24. A region of reflective multilayer 20 may beaffected by depositing energy within the region. For example, the regionmay be irradiated by an treatment beam 36, e.g., produced by a laser orotherwise, or otherwise locally heated.

Treatment beam 36 may be produced by treatment radiation source 32. Forexample, treatment radiation source 32 may include or represent anultrashort pulsed laser that produces picosecond pulses or shorter. Theultrashort pulsed laser may be a femtosecond pulsed laser. Treatmentbeam 36 may be focused, directed, collimated, or otherwise modified bybeam optics 34. For example, beam optics 34 may direct treatment beam 36to scan across a region of reflective multilayer 20 in a predeterminedscan pattern, e.g., in a raster pattern. Such a raster pattern may becharacterized by such parameters as a pitch (e.g., distance betweensequential laser spots) and a scan rate. Such a raster pattern may beconfigured to deposit a quantity of energy within the region that issufficient to produce a treated multilayer region 24.

For example, the reflectance of reflective multilayer 20 prior totreatment may be about 0.7. A treatment to change the reflectance ofreflective multilayer 20 may include irradiation of reflectivemultilayer 20 with laser radiation having an energy density in the rangeof from about 10 mJ/cm² to about 10 J/cm². For example, in order for thereflectance to be changed from an initial value of about 70% to about40% (a change of 30%), an energy density of about 2 J/cm² may beapplied. In some cases, an appropriate compensating change inreflectance may be less than 3%. In such a case, the energy density ofthe irradiation may be about 200 mJ/cm² or less.

In order for a smooth and homogeneous change in the reflectance to beproduced, a sequence of overlapping laser spots may be scanned over thesurface of reflective multilayer 20 in the form of a raster pattern. Theraster pattern may be characterized by a ratio of laser spot diameter topitch. For example, the ratio of spot diameter to pitch may be in therange of from about 1 to about 100. In some cases, a pattern ofnon-overlapping laser spots may be applied.

Treatment beam 36 is directed at various regions of reflectivemultilayer 20. For example, one or more of treatment radiation source32, beam optics 34, or reflective photomask 19 (or a stage or platformupon which reflective photomask 19 is mounted) may be configured to movewith a one- or two-dimensional translational motion (schematicallyrepresented by translation motion 46) relative to one or more othercomponents of photomask treatment system 30.

Operation of one or more components or operations of photomask treatmentsystem 30 may be controlled by controller 38. For example, controller 38may control operation of one or more of treatment radiation source 32(e.g., turning on or off, pulse rate or energy, wavelength range, orother parameter of operation of treatment radiation source 32), beamoptics 34 (e.g., focus, aperture, scanning rate, raster patternparameters, or other operation of beam optics 34, translation motion 46(e.g., of a translation mechanism), or other operation of photomasktreatment system 30.

Controller 38 may include a processor 40. Alternatively or in addition,controller 38 may include digital or analog circuitry that is configuredto control operation of one or more components of photomask treatmentsystem 30.

For example, processor 40 may include one or more processing units,e.g., of one or more computers. One or more components of processor 40may be incorporated into one or more components of photomask treatmentsystem 30. Processor 40 may be configured to operate in accordance withprogrammed instructions.

Processor 40 may communicate with input/output unit 44. Input/outputunit 44 may include a computer monitor or screen. Processor 40 maycommunicate with input/output unit 44 to display a status of one or morecomponents of photomask treatment system 30. In another example,input/output unit 44 may include a printer, display panel, speaker, oranother device capable of producing visible, audible, or tactile output.

Input/output unit 44 may include one or more user-operable controls toenable a user or operator of photomask treatment system 30 to start,stop, or otherwise control operation of one or more components ofphotomask treatment system 30. For example, an input device ofinput/output unit 44 may include one or more of a keyboard, keypad,pointing device, touch screen, or control panel for enabling a user toinput commands, data, or instructions for operation of processor 40.

Input/output unit 44 may enable communication with, or transfer of datato or from, one or more other devices. For example, input/output unit 44may enable communication with a system for measuring CD variation duringuse of reflective photomask 19 in photolithography. Input/output unit 44may enable receiving data from inspection unit 29 of EUVphotolithography system 10 (FIG. 1).

Processor 40 may communicate with data storage unit 42. Data storageunit 42 may include one or more fixed or removable, volatile ornonvolatile, memory or data storage devices. Data storage unit 42 may beutilized to store, for example, programmed instructions for operation ofprocessor 40, data or parameters for use by processor 40 duringoperation, or results of operation of processor 40. For example, datastorage unit 42 may be utilized to store results of inspection of awafer print that is produced using reflective photomask 19, andparameters related to treatment of reflective multilayer 20 to produce atreated multilayer region 24.

Alternatively or in addition to direct irradiation of reflectivemultilayer 20 by treatment beam 36, reflective multilayer 20 may betreated by a treatment beam that traverses photomask substrate 26.

FIG. 2B schematically illustrates components of a variation of thesystem of FIG. 2A.

In alternative photomask treatment system 31, treatment beam 36traverses photomask substrate 26 before impinging on reflectivemultilayer 20. In this manner, treatment beam 36 may avoid irradiatingthe absorber pattern that is on the exposed side of reflectivemultilayer 20. Alternative photomask treatment system 31 may beconfigured that treated multilayer region 24 is located at a finitedepth within reflective multilayer 20 (e.g., close to photomasksubstrate 26). Such irradiation via photomask substrate 26 may enablefiner control of treatment results than direct irradiation of exposedmultilayer surface 21. For example, a wavelength of treatment radiationsource 32 may be selected such that treatment beam 36 may traversephotomask substrate 26 without significant absorption, but is stronglyabsorbed by reflective multilayer 20. In some cases, an exposed surfaceof photomask substrate 26 may include a coating that is transparent totreatment beam 36.

Heating or other treatment of a region of reflective multilayer 20 toform treated multilayer region 24 may affect the reflectance of treatedmultilayer region 24.

FIG. 3 schematically illustrates modification of reflectance of a regionof a reflective multilayer that is treated to correct for CD variation,in accordance with an embodiment of the present invention.

Graph 50 shows various curves representing spectral reflectance (R) of aregion of a reflective multilayer as a function of wavelength (λ).Working wavelength 52 represents a wavelength at which an EUVphotolithography system operates, and at which the reflectance of theregion of the reflective multilayer is of interest. The reflectivemultilayer, prior to treatment, may be configured for optimal or maximumreflectance at working wavelength 52.

Spectral reflectance curve 54 schematically represents a spectralreflectance of an untreated region of a reflective multilayer (forsimplicity, only the central first order peak of the reflectance curveis schematically represented). Spectral reflectance curve 54 isoptimized for maximum reflectance value 53 (e.g., about 70% or anothervalue) at working wavelength 52.

Reduced spectral reflectance curve 56 schematically represents aspectral reflectance of a treated region of a reflective multilayer.Reduced spectral reflectance curve 56 may result from a treatment of thereflective multilayer that reduces the local reflectance withoutaffecting the wavelength dependence of the reflectance. The reflectanceremains optimized for working wavelength 52. However, peak reflectancevalue 55 of reduced spectral reflectance curve 56 is less than maximumreflectance value 53 of spectral reflectance curve 54. Similarly, thereflectance value at all wavelengths is reduced relative to spectralreflectance curve 56.

Shifted spectral reflectance curve 58 schematically represents aspectral reflectance of a treated region of a reflective multilayer.Shifted spectral reflectance curve 58 may result from a treatment of thereflective multilayer that shifts the optimum wavelength of the localreflectance without reflectance values. The reflectance of shiftedspectral reflectance curve 58 is optimized for shifted wavelength 59.However, reflectance value at shifted wavelength 59 remains equal tomaximum reflectance value 53 of spectral reflectance curve 54. However,due to the wavelength shift, the reflectance value of shifted spectralreflectance curve 58 at working wavelength 52 is now equal to off-peakreflectance value 57.

Although in the example of shifted spectral reflectance curve 58,shifted wavelength 59 is shown at a shorter wavelength than workingwavelength 52, the shift may be in the opposite direction (with themaximum of the shifted curve at a longer wavelength than workingwavelength 52). It may also be noted that all or part of a reflectivemultilayer may have been non-optimally fabricated such that the maximumreflectance is not at working wavelength 52 (e.g., similar to shiftedspectral reflectance curve 58). In such a case, an appropriate treatmentof the reflective multilayer may shift the spectral reflectance suchthat the maximum reflectance is shifted to the working wavelength (e.g.,as in spectral reflectance curve 54), thus increasing the reflectance atworking wavelength 52 (e.g., from off-peak reflectance value 57 tomaximum reflectance value 53).

It may be noted that a treatment of a region of the reflectivemultilayer may both change a maximum value of the spectral reflectancecurve and shift the maximum of the spectral reflectance curve to anotherwavelength. Thus, the effects that are illustrated by reduced spectralreflectance curve 56 and by shifted spectral reflectance curve 56 mayboth be present in the treated region.

Treatment parameters for treating a region of a reflective multilayer ofa reflective photomask for EUV photolithography may be determined by acalibration process. The calibration process may include treating aplurality of regions of a reflective multilayer. Different regions, orgroups of regions, may be treated in accordance with different treatmentparameters. The regions may be examined after the treatment to determinethe effect of each set of treatment parameters on reflectance. Theresults of the measurements may be stored in a database or lookup tablefor later reference.

When a reflective multilayer is to be treated for CD variationcorrection, a change in reflectance of a region of the reflectivemultilayer that corrects the CD variation may be calculated. The storedmeasurement results may be searched to identify a set of parameters thatproduces the calculated change in reflectance. The region may then betreated in accordance with the identified parameters.

FIG. 4 schematically illustrates a reflective multilayer surface forcalibration of treatment to correct the CD variation, in accordance withan embodiment of the present invention. In discussing FIG. 4, referenceis also made to items of FIG. 2.

Each cell 62 of multilayer surface 60 (in the form of a reflectivemultilayer 20) may be treated. A cell may represent a treated area ofany shape or form (and not necessarily square as shown). A position ofeach cell may be determined by a coordinate system. For example, acoordinate system may be defined relative to one or more fiducial marks64.

For example, multilayer surface may be positioned in place of reflectivelayer 20 in photomask treatment system 30. Multilayer 60 may be movedwith translation motion 46 such that each cell 62 may be irradiated bytreatment beam 36. During irradiation, treatment radiation source 32 orbeam optics 34 may be operated such that different cells 62 areirradiated in accordance with different treatment parameters. Variabletreatment parameters may include, for example, one or more of pulseenergy, pulse duration, pulse rate, scan rate of raster pattern, pitchof raster pattern, total energy deposited, rate of energy deposition, oranother treatment parameter. Variable treatment parameters may definefocusing conditions (e.g., focal length, relative distances, or otherparameters that affect optics or focusing conditions). Treatmentparameters for each cell 62 may be stored, e.g., on data storage unit42, together with coordinates of the corresponding cell 62.

After treatment, each cell 62 may be examined to determine the effect onproperties of that cell 62. Various examinations may be automatic or mayrequire involvement by a human observer.

A measurement of EUV reflectance of a cell 62 at the working wavelengthmay determine a change in reflectance at the working wavelength. Ameasurement of EUV spectral reflectance may determine if the change inreflectance results from a reduction of peak reflectance or results froma wavelength shift of the reflectance spectrum. A change in reflectancemay be stored together with the corresponding treatment parameters in adatabase or lookup table. The database or lookup table may be consultedwhen treating a reflective multilayer 20 to form a treated multilayerregion 24 with a change in reflectance configured to correct CDvariation.

A method of treating a reflective multilayer 20 to form one or moretreated multilayer regions 24 may be executed by a processor 40 ofcontroller 38 of a photomask treatment system 30.

FIG. 5 is a flowchart illustrating a method for correcting CD variationsin EUV lithography, in accordance with an embodiment of the presentinvention.

It should be understood, with respect to any flowchart referencedherein, that the division of the illustrated method into discreteoperations represented by blocks of the flowchart has been selected forconvenience and clarity only. Alternative division of the illustratedmethod into discrete operations is possible with equivalent results.Such alternative division of the illustrated method into discreteoperations should be understood as representing other embodiments of theillustrated method.

Similarly, it should be understood that, unless indicated otherwise, theillustrated order of execution of the operations represented by blocksof any flowchart referenced herein has been selected for convenience andclarity only. Operations of the illustrated method may be executed in analternative order, or concurrently, with equivalent results. Suchreordering of operations of the illustrated method should be understoodas representing other embodiments of the illustrated method.

CD variation correction method 100 may be executed by a controller of aphotomask treatment system. For example, CD variation correction method100 may be executed by a processor of the controller that is operatingin accordance with programmed instructions. CD variation correctionmethod 100 may be performed on reflective multilayer of a reflectivephotomask for EUV photolithography.

CD variation correction method 100 may be executed when a reflectancechange that is required to correct CD variation has been calculated as aresult of mapping CD variation (block 110). For example, a calculationof a required reflectance change may have been calculated on the basisof results of EUV photolithography using the reflective photomask. Aresult of the EUV photolithography, or a photomask itself, may have beeninspected by an appropriate inspection device to detect CD variation. Areflectance change to correct the detected CD variation may have beencalculated. For example, the reflectance change may be calculated on thebasis of a known relationship between an exposure of a photoresist toEUV radiation and a threshold for activation of the photoresist (e.g.,the activation threshold varying linearly with the logarithm of theexposure dose). Thus, if the detected CD variation is above a specifiedvalue, a reduction of the EUV exposure may change the CD value, thusreducing a width of a printed line or diameter of a hole to, or closerto, the specified value.

Treatment parameters for producing the determined reflectance change maybe determined (block 120). Results of a calibration may be utilized toidentify the treatment parameters that are required to produce thedetermined reflectance change. For example, a lookup table or databasethat was generated or modified during a calibration operation a regionof a reflector of EUV photomask to correct CD variation may be utilizedto identify treatment parameters to produce the determined reflectancechange. In some cases, treatment parameters that correspond to thedetermined reflectance change may not be present in the calibrationtable or database. In this case, interpolation among, or extrapolationfrom, treatment parameters for similar reflectance changes may beutilized to calculate the required treatment parameters.

A treatment beam may be directed to a region of the reflectivemultilayer that is to be treated (block 130). For example, a laterallytranslatable stage of the radiation source or of the reflectivephotomask may be moved to bring the treatment radiation source to thevicinity of the region to be treated. Beam optics may be controlled toaim the treatment beam at the region to be treated.

The region may be treated in accordance with the determined treatmentparameters (block 140). For example, a CW or pulsed laser (includingultrashort pulsed laser) beam may be operated to irradiate the regionwith a predetermined quantity of radiation energy during a predeterminedtime interval with a predetermined distribution over the area. Forexample, the beam may be scanned across the surface of the region, e.g.,in a raster pattern. The beam may be pulsed in a predetermined manner(e.g., pulse energy, duration, rate, total exposure time, or other pulseparameters).

Typically, a reflective photomask includes a plurality of dies, oractive areas. Each die is spatially separated from each adjacent die byborder, typically referred to as a black border. The black border mayinclude a layer of a material that absorbs EUV radiation. In some cases,the black border may have a width of about 2 mm to about 3 mm. In somecases, the absorbing layer may have a thickness of about 50 mm to about70 mm. (Additional thickness may be precluded by a desire to avoid orminimize shadowing by the absorbing layer.) The layer of EUV absorbingmaterial is typically deposited onto the reflective multilayer of theEUV photomask.

Although ideally the black border should not reflect any EUV radiation,in practice, a typical black border may reflect up to 3% of incident EUVradiation. Much of this reflection is due to reflection by thereflective multilayer of the photomask of radiation that is not absorbedby the EUV absorbing layer. Reflection by the black border could causeunacceptable CD changes and losses of contrast in regions of a die inthe vicinity of the black border. In some cases, a region of the EUVabsorbing layer may be defective (e.g., may include a hole or otherregion of reduced absorbance) so as to enable increased (over otherregions of the EUV absorbing layer) transmission of EUV radiation to thereflective multilayer.

In accordance with an embodiment of the present invention, thereflectivity of the black border may be reduced by treating thereflective multilayer under the absorbing layer of the black border byheating. For example, radiation from a suitable radiation source, suchas a CW laser, a pulsed laser (e.g., an ultrafast, e.g., femtosecond,pulsed laser), or another suitable radiation source. The treatment ofthe reflective multilayer at the black border may lower the reflectanceof the multilayer to a predetermined reflectance value or less. Forexample, in the case of treatment at the black border, the appliedtreatment may include deposition of sufficient energy so as to reducethe reflectance of the reflective multilayer to a reflectance value ofno more than 5% (e.g., from an initial value of about 60% to 67%, e.g.,65%, to a final value of about 5%, or from another initial value toanother final value). As a result, the reflectance of the black bordermay be reduced to an acceptable value (e.g., from an initial reflectanceof about 2.6% prior to treatment, e.g., where the transmittance of theabsorbing layer is about 20%, to a final value of about 0.2% aftertreatment, or from another initial value to another final value).

Reducing reflectance of the reflective multilayer at a black border byradiation heating may be advantageous over other described techniques.For example, radiation treatment to reduce the reflectance may beadvantageous over etching away (the absorber and) the multilayer at theblack border since such etching could be difficult (e.g., requiringetching to a depth of about 300 nm or more), could promote degradationof the photomask (e.g., during cleaning), or could lead to unwantedelectrical charging effects.

FIG. 6 schematically illustrates a layout of a EUV photomask with ablack border to which a reflection reducing treatment is applicable, inaccordance with an embodiment of the present invention.

EUV photomask 70 includes a plurality of dies 72 that are separated byblack borders 74.

FIG. 7 schematically illustrates application of a treatment for reducingreflectance of a black border, in accordance with an embodiment of thepresent invention.

Reflective photomask 86 includes a black border 81 that separatesphotomask dies 88. Black border 81 includes an absorbing layer 82 thatis deposited on reflective multilayer 20.

In black border treatment system 80, treatment beam 36 deposits heatingradiation within treated multilayer region 84 of reflective multilayer20. The energy that is deposited in treated multilayer region 84 reducesthe reflection of EUV radiation by treated multilayer region 84. Theresulting reduction in reflectance of treated multilayer region 84 maybe sufficient to reduce reflectance of black border 81 to an acceptablelevel.

In some cases, treated multilayer treatment beam 36 may be limited to aregion of reflective multilayer 20 that is in the vicinity of a knowndefect in absorbing layer 82. For example, a defect may have beendetected using one or more inspection or testing techniques.

In some cases, treatment beam 36 may traverse a backside coating layer89 and photomask substrate 26. For example, backside coating layer 89may include chromium nitride (CrN).

FIG. 8 is a flowchart illustrating a method for reducing reflectance ofa black border, in accordance with an embodiment of the presentinvention.

Black border treatment method 200 may be executed by a controller of aphotomask treatment system. For example, black border treatment method200 may be executed by a processor of the controller that is operatingin accordance with programmed instructions. Black border treatmentmethod 200 may be performed on a black border of a reflective photomaskfor EUV photolithography.

A treatment beam is directed to the reflective multilayer at the blackborder (block 210). For example, a laterally translatable stage of theradiation source or of the reflective photomask may be moved to bringthe treatment radiation source to the vicinity of the black border. Beamoptics may be controlled to aim the treatment beam at the reflectivemultilayer at the black border. In some cases, the treatment beam may bedirected to the reflective layer at a region of the black border thathas been found to be defective.

The treatment beam may be operated to irradiate the reflectivemultilayer at the black border in order to reduce the reflectance of thereflective multilayer at the black border (block 220). For example, a CWor pulsed laser (including ultrashort pulsed laser) beam may be operatedto irradiate the reflective multilayer at the black border with apredetermined quantity of radiation energy during a predetermined timeinterval with a predetermined distribution over the area. For example,the beam may be scanned across the surface of the region, e.g., in araster pattern. The beam may be pulsed in a predetermined manner (e.g.,pulse energy, duration, rate, total exposure time, or other pulseparameters).

Different embodiments are disclosed herein. Features of certainembodiments may be combined with features of other embodiments; thuscertain embodiments may be combinations of features of multipleembodiments. The foregoing description of the embodiments of theinvention has been presented for the purposes of illustration anddescription. It is not intended to be exhaustive or to limit theinvention to the precise form disclosed. It should be appreciated bypersons skilled in the art that many modifications, variations,substitutions, changes, and equivalents are possible in light of theabove teaching. It is, therefore, to be understood that the appendedclaims are intended to cover all such modifications and changes as fallwithin the true spirit of the invention.

While certain features of the invention have been illustrated anddescribed herein, many modifications, substitutions, changes, andequivalents will now occur to those of ordinary skill in the art. It is,therefore, to be understood that the appended claims are intended tocover all such modifications and changes as fall within the true spiritof the invention.

1. A system for correcting a CD variation in EUV photolithography, thesystem comprising: a radiation source to produce a treatment beam; and acontroller configured to direct the treatment beam at a region of areflective multilayer of an EUV photolithography photomask at a blackborder of the photomask, and to operate the source to treat the regionwith the beam in accordance with one or a plurality of parameters toreduce a reflectance of the black border at a working wavelength of EUVradiation in the region to at least a predetermined reflectance value.2. The system of claim 1, wherein the treatment beam is configured toirradiate the region via a substrate and backside coating of thephotomask.
 3. The system of claim 1, further comprising opticsconfigured to scan the beam over the region.
 4. A method for reducingreflection by a black border of a photomask for EUV photolithography,the method comprising: directing a treatment beam from a radiationsource to a reflective multilayer of the photomask at the black border;and treating the reflective multilayer at the black border with thetreatment beam so as to reduce reflection of the multilayer to at leasta predetermined reflectance value.
 5. The method of claim 4, wherein thetreatment beam is directed to the reflective multilayer through abackside coating and a photomask substrate.
 6. The method of claim 4,wherein the predetermined reflectance value is 5%.
 7. The method ofclaim 4, wherein the treatment beam heats the reflective multilayer. 8.The method of claim 4, wherein directing the treatment beam comprisesdirecting the treatment beam to the reflective multilayer at a defectiveregion of the black border.